Floppy disk subsystem, user's manual (Engineering number 91-53566-00A0, January 1977)
The year should probably be 1976, not 1977.

Floppy-disk subsystem, user's manual (38 MB) (Engineering number 91-53566-00A2, September 1976)
No cover. Missing one or two of the first pages. Likely to be A2 version the A3 version above. Matches A2 version description in A3 version.